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word tracker and link builder can help you greatly. http://www.wordtracker.com/

Hi Liors, I'm going through older unanswered questions here in Q&A. Did those suggestions meet your needs, or are you still looking for some more advice about Google Shopping?

Really both. On the one hand, the value of building many links on the same domain diminishes as the number of links on that domain increases. However, in my experience, this doesn't start happening until you're talking a lot of links (think footer links & blog roll links). Going after the new domain will increase the domain diversity of your back link profile, which is really important. Also, are we assuming that both domains are of equal authority? My advice would be to continue spending time building links on domain #1 and start investing time into the more difficult domain #2.